This invention relates to a method and apparatus for diluting an aerosol stream having a high particulate concentration so that subsequent particle measurement is more accurate. It also relates to the use of a diluter in combination with a Condensation Particle Counter (CPC) to measure aerosols at high concentrations.
Aerosols consisting of small particles suspended in air are widely encountered in nature and in the human environment. Techniques for aerosol measurement are important for studying the behavior of aerosols and their effect on the environment in which they occur. An important technique for aerosol measurement is the Optical Particle Counter (OPC). The instrument is also referred to as a Laser Particle Counter (LPC) if a laser light source is used for particle illumination. Another technique is the Condensation Particle Counter (CPC). Both techniques involve optical detection based on light-scattering, with the OPC detecting the particle by direct light scattering from the particle while the CPC detecting the particle by first condensing a vapor on the particle to form a droplet, which is then detected optically by light scattering or other techniques.
The lower detection of the OPC is generally on the order of 0.1 μm in particle diameter. Smaller particles can be detected by the OPC by using a stronger laser light source, a more sensitive photo-detector, or both, but the apparatus becomes increasingly more difficult to design and costly to manufacturer, making an alternative method of measurement preferred. An alternative device is the CPC that is capable of detecting particles as small as 0.002 μm in diameter. Very small particles can be detected by the CPC because a vapor is first condensed on the particles to form droplets of a larger size. The droplets are then detected optically by light scattering or other techniques.
Both the OPC and CPC are useful instruments for aerosol measurement with the CPC particularly useful for measuring small particles below the lower detection limit of the OPC, which is generally about 0.1 μm. Because of the potential health effect of small particles emitted by Diesel and spark ignition engines and other combustion sources, CPC is becoming increasingly important as a measuring instrument for engine exhaust particulate and similar measurements.
Historically the Condensation Particle Counter (CPC) is referred to as a Condensation Nucleus Counter (CNC) because the particles being detected form the nucleus of condensation in droplet formation. Both CPC and CNC are now used in the scientific and technical literature to refer to a measuring instrument based on vapor condensation, droplet growth and optical detection.
Both the CPC and the OPC have certain inherent limits on particle concentration. The count rate limit of a particle counter is exceeded when particles are passing through the light beam at too rapid a rate for the particles to be counted reliably by the detecting and counting circuitry. The coincidence counting limit is exceeded when more than one particle is present in the optical view volume of the detector causing light scattering from two or more particles to appear as one, leading to losses in particle counts. For high accuracy measurement both of these limits must not be exceeded.
For aerosol measurement by the CPC above the coincidence counting limit a photometric mode is often used. In the photometric mode, light scattering from the droplet cloud illuminated by the light beam is measured by the optical detector and used as a measure of droplet concentration. One commercial instrument using the photometric mode is the Model 3022A CPC from TSI, Inc. In this instrument, aerosols up to ˜104 particles per cc are measured by single particle counting, while the photometric mode is used for high concentrations up to a maximum limit of ˜107 particles per cc in concentration.
Since light scattering from a droplet cloud depends both on droplet size and droplet concentration, the photometric mode is less accurate than the single particle counting mode. In the photometric mode, a small variation in droplet size due to variation in vapor saturation and condensation conditions can cause the droplet size to change, giving rise to different instrument readings even if the aerosol concentration remains the same. A recent study shows that when several CPCs are placed in a side by side comparison, differences as much as 60% or more in the measured aerosol concentration can occur. (“Performance Evaluation of a Recently Developed Water-Based Condensation Particle Counter,” S. Biswas, P. M. Fine, M. D. Geller, S. V. Hering and C. Sioutas, Aerosol Science and Technology, Vol. 39, pp. 419-427, 2005.) The photometric mode is believed to have contributed significantly to the observed discrepancy in the measured concentration.